Astrophysics

Title:
How Mira Variables Change Visual Light by a Thousand-fold

Abstract: Mira variables change visual light by up to 8 magnitudes over their roughly
yearly cycle. Here we present a simple explanation for the extremely large
amplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, such
as TiO, form throughout the stellar atmosphere as the star cools when
approaching minimum light. When this happens, the visual light can be almost
completely absorbed at large radii, extending the visual photosphere to nearly
twice its nominal size. At these large radii, temperatures can fall to
approximately 1400 K and essentially all of the star's radiation emerges in the
infrared. Since almost no optical light is emitted at these low temperatures,
Mira variables can decrease their visual light by more than a thousand-fold and
almost "disappear" to the human eye.